The use of a differential reflective index between a crystalline phase and an amorphous phase in an optical information storage medium was originated from Ovshinsky, et al. in 1970. Te alloys were the primary research subjects initially. The element Te is easy in forming an amorphous phase, but it has a crystallization temperature of only 10° C. and a rapid crystallization rate, which cause an unstable amorphous phase. In order to enhance the stability of the amorphous phase, other elements have been incorporated to form Te alloys. Dozens of alloy materials suitable for being used as an optical recording material have been made by researchers over the past 30 years. These materials include GeTe, GeTeS, SbSe, SbTe, BiTe, TeSeSbS, GeSnTe, TeSeGe, TeOInGeAu, SbSeBi, InSb+M, GaSbTe, TeSe+M, TeGeSbSe, GeSbTe, InSbTe, InSbSe, InTeSe, AgInTe, and AgInSbTe, etc. Up to now, however, only two major series of materials (GeSbTe and AgInSbTe) are phase change materials that are commercially feasible in the production of rewritable optical discs.
Furthermore, since the magnitude of the focal point of a laser light is proportional to the wavelength of the laser light, a recording density is inversely proportional to the wavelength of the laser light used. The CD series of optical discs adopt a near infrared (IR) light with a wavelength of 780 nm and have a capacity of 650 MB; and the DVD series of optical discs adopt an IR light with a wavelength of 635˜650 nm and have a capacity of 4.7 GB. As for the next generation HD-DVD series of optical discs having a capacity of over 15 GB, a blue laser light with a wavelength of about 400 nm will be used. Thus, the search for phase change materials suitable for a blue laser light has become a major task in the development for a HD-DVD rewritable optical disc. Since 1999, major manufacturers of optical discs from Japan and Europe have been publishing phase change materials suitable for a blue laser light in major international conferences. Most of these materials are GeSbTe stoichiometric compound series and doped Sb69Te31 eutectic alloy series derived from those commonly used in the current Cd and DVD discs. These materials include GeSbTe, GeSbSnTe, Ge+ doped eutectic Sb69Te31, AgInSbTe, Ge(Sb69Te31)+Sb and AgInSbTeGe, etc.
A GeSb binary alloy has an eutectic composition of Ge14.5Sb85.5 and an eutectic temperature of 592° C. When the temperature is lower than the eutectic temperature, the crystal forms include only the individual forms of Ge and Sb and are free of any other form. J. M. del Pozo, et al. [J. Non-cryst. Sol., Vol. 185 (1995) 183] have discovered that a GeSb alloy contains a gradually reduced amount of Ge crystals when the Sb concentration thereof increases gradually. Once the Sb concentration of the GeSb alloy exceeds that of the GeSb eutectic alloy, the crystallization behavior of the GeSb alloy is like pure Sb. C. N. Afonso, et al. [Appl. Phys. Lett. Vol. 60 (1992) 3123] have discovered that the crystallization temperature of GeSb alloy decreases gradually along with an increase in the Sb content therein, but remains higher than 150° C., while the activation energy thereof remains higher than 1.5 eV. When a GeSb film is written or erased by an extremely short laser pulsation (ps) and a higher energy density (13-56 mJ/cm2), a rapid amorphous-crystalline phase change and a higher contrast of reflective index can be obtained. However, the research of C. N. Afonso, et al. is a basic study on a single GeSb film layer. Moreover, an erase operation performed by a short laser pulsation and a high energy density used by the basic research is likely to cause a recording layer to form a partial crystallization and consequently reduce the modulation. Another major defect for a GeSb binary alloy being used as a recording material is that such an alloy has an excessively large grain size (about 0.1˜5 μm) which renders said alloy unsuitable in optical recording. An ordinary CD has a recording track of about 1 μm, a blue laser light has a minimum recording track of about 0.21 μm, and a red light near-field recording has a minimum recording track of less than about 0.21 μm. In the latter two applications, a grain size of about 0.1˜5 μm will cause an increased jitter.